1. Field of the Invention
The present invention relates generally to Embeddable Corrosion Rate Meter (ECRM) instrumentation for remote monitoring of structures susceptible to corrosion.
2. Description of the Related Art
The basic operating principles of the Embeddable Corrosion Rate Meter (ECRM) instrumentation works on the principles of a technique known as chronovoltammetry, i.e., Voltage-Time Response. However, recent changes to the instrumentation part of the ECRM provides the flexibility of using principles of yet another technique, known as alternating current (AC) impedance or Electrochemical Impedance Spectroscopy (EIS), to estimate corrosion rates. The ECRM instrumentation or sensor contains a test electrode that is perturbed or excited with one or more current (I) pulses. The time-dependent changes (response) in the electrochemical potential (Y) of the electrode are measured.
Alternatively, a set of constant potential pulses can be used as the perturbation signal to measure the resulting current transients (chronoamperommetry) and estimate the corrosion rates. This notwithstanding, the rest of the application is directed to chronovoltammetry. The ECRM instruments or sensors are small, comparable in size to concrete aggregates and require very little electric power to operate. The electronic circuit necessary for making the instrument is relatively simple. The use of chronovoltammetry allows ECRM instruments to work in electrolytes, such as concrete, that are not good conductors of electricity.
The (V/I) ratio, also known as the polarization resistance, Rp, is inversely proportional to the corrosion rate. The conventional corrosion rate measurement techniques such as linear polarization and logarithmic polarization also estimate Rp described in “Testing of Concrete in Structures”, Ed. J. H. Bungey and S. G. Millard, Blackie Academic & Professional, NY, Third Edition, 1996, p. 173. These techniques use a direct current (DC) or voltage source to perturb the electrode and measure the DC voltage or current response using relatively simple electronic circuitry. However, the conventional corrosion rate measurement techniques are not useful in measuring corrosion rates when the metal is in contact with mediums that are poor conductors of electricity. These techniques suffer from an error caused by the resistive drop, also known as “IR-Drop”, that occurs when the current passes through the resistive medium. Therefore, the use of linear and logarithmic polarization techniques could result in erroneous estimation of corrosion rates.
There are also techniques based on alternating current (AC) principles. For example, AC impedance or electrochemical impedance spectroscopy (EIS) can measure Rp more accurately than the DC techniques, but it requires complex electronic circuits. The chronovoltammetry-based ECRM employs a relatively simple electronic circuit, overcomes the problem of IR-Drop, can be designed to be small, and requires very little power to operate. ECRM, which is an ideal corrosion rate meter, is embeddable in concrete or soil to measure corrosion rates of steel reinforcing bars (rebars), pipelines, and other buried structures.
Similar to linear and logarithmic polarization, and EIS techniques, the ECRM also uses principles of electrochemistry to measure corrosion rates. In essence, all electrochemical techniques apply a known voltage to the metal under test, and measure the resulting current flow across the metal/electrolyte (concrete) interface. Alternatively, in some cases, the perturbing signal is a known current, and the resulting change in the voltage across the metal/electrolyte interface is measured; the resistance across the electrode/electrolyte interface is the polarization resistance, Rp. The current-voltage relationship provides the rate of corrosion of the metal in the medium (concrete).
A major problem with most techniques is the electrical resistance of the concrete: the current that flows through the concrete generates a voltage drop, Vconc=IRconc (IR-Drop) across its resistance. Thus, the voltage applied or measured is V, which is the sum of IRconc and IRp; IRconc=Vconc; IRp=Vp; and V=Vconc+Vp. In concrete, Rconc can be much larger than Rp, and unless the correction is made for the voltage drop, Vconc, across Rconc, the corrosion rate will be grossly underestimated. Most electrochemical techniques suffer from this limitation, and some of them use chronovoltammetry for the IR-Drop correction. In other words, they combine chronovoltammetry for IR-Drop correction with yet another technique to measure the rate of corrosion. An obvious, practical limitation is using at least two types of electronics and instrumentation, one for IR-Drop correction, and another for corrosion rate measurement.